Wind turbine blade with optical sensor system

ABSTRACT

Wind turbine blade comprising a sensor system with an optical path comprising a first optical sensor fiber, a second optical sensor fiber and a patch optical fiber, the first optical sensor fiber including a first core with a first core diameter wherein the first optical sensor fiber extends from a first end to a second end and comprising at least one sensor, the second optical sensor fiber including a second core with a second core diameter, wherein the second optical sensor fiber extends from a first end to a second end and comprising at least one sensor, the patch optical fiber including a patch core with a patch core diameter, wherein the patch optical fiber extends from a first end to a second end and connects the first optical sensor fiber and the second optical sensor fiber, wherein the first core diameter is the same as the patch core diameter.

This is a National Phase Application filed under 35 U.S.C. §371 as anational stage of International Application No. PCT/EP2011/073249, filedon Dec. 19, 2011, claiming the benefit from European Patent ApplicationNo. 10197351.9, filed on Dec. 30, 2010, the content of each of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a wind turbine blade, and a method andapparatus for determining or estimating loads such as bending moment ofa wind turbine blade, in particular root moment near or at the root endof a wind turbine blade.

BACKGROUND

Wind turbine manufacturers are constantly making efforts to improve theefficiency of their wind turbines in order to maximise the annual energyproduction. Further, the wind turbine manufacturers are interested inprolonging the lifetime of their wind turbine models, since it takes along time and a lot of resources to develop a new wind turbine model.Systems for monitoring operating parameters of a wind turbine and itscomponents such as the wind turbine blades have become an area ofincreased attention in order to optimize performance and prolonging thelifetime of different components.

Accordingly, there is a need for a wind turbine blade, methods andapparatus enabling accurate and efficient monitoring of one or moreoperating parameters of a wind turbine or wind turbine blade. An area ofparticular interest may be monitoring the loads and stresses applied ona wind turbine blade during operation of a wind turbine.

DISCLOSURE OF THE INVENTION

Accordingly, the invention provides a wind turbine blade extending alonga longitudinal axis from a root end to a tip end and in a transverseplane perpendicular to the longitudinal axis, the transverse planehaving a main axis extending through an elastic center point, the windturbine blade comprising a blade shell forming a profiled contourincluding a pressure side and a suction side, as well as a leading edgeand a trailing edge with a chord having a chord length extending therebetween, the main axis being parallel to the chord. The wind turbineblade may include a sensor system comprising a first sensor set, e.g.for measuring a first bending moment, in a first sensor position at afirst distance from the root end, the first sensor set comprising afirst primary sensor for measuring a primary component and a firstsecondary sensor for measuring a secondary component, wherein a firstprimary sensor axis in the transverse plane is oriented in a directiondefined by the first primary sensor and the elastic center point, and afirst secondary sensor axis in the transverse plane is oriented in adirection defined by the first secondary sensor and the elastic centerpoint. An angle between the first primary sensor axis and the firstsecondary sensor axis may be in the range from 50° to 130°.

Accordingly, the wind turbine blade according to the invention allowsdetermination of bending moment with only two sensors in a cross-sectionsaving manufacture costs.

Also disclosed is a wind turbine blade extending along a longitudinalaxis from a root end to a tip end and in a transverse planeperpendicular to the longitudinal axis, the transverse plane having amain axis extending through an elastic center point, the wind turbineblade comprising a blade shell forming a profiled contour including apressure side and a suction side, as well as a leading edge and atrailing edge with a chord having a chord length extending therebetween, the main axis being parallel to the chord is provided. The windturbine blade may comprise a sensor system including a plurality ofsensor sets, each sensor set comprising a plurality of sensors includinga primary sensor and a secondary sensor for measuring a primarycomponent and a secondary component, respectively. The plurality ofsensor sets includes a first sensor set for measuring a first bendingmoment in a first sensor position at a first distance from the root end,and a second sensor set for measuring a second bending moment in asecond sensor position different from the first sensor position at asecond distance from the root end. The first distance may be at least 1m. The second distance may be at least 3 m.

Also disclosed is a method for estimating bending moment of a windturbine blade, e.g. of a wind turbine blade as disclosed herein. Thewind turbine blade extends along a longitudinal axis from a root end toa tip end and having a root region, a transition region and an airfoilregion, the method comprising the steps of obtaining a first sensor setsignal indicative of a first bending moment at a first sensor positiondifferent from the tip end along the longitudinal axis of the windturbine blade, and estimating a bending moment at a first estimationposition along the longitudinal axis based on the first sensor setsignal, wherein the first sensor position is different from the firstestimation position along the longitudinal axis.

Further, a blade monitoring system for monitoring a wind turbine bladecomprising a sensor system is disclosed, the blade monitoring systemcomprising a processing unit and an interface connected to theprocessing unit. The processing unit is configured for receiving a firstsensor set signal indicative of a first bending moment at a first sensorposition of a wind turbine blade extending along a longitudinal axisfrom a root end to a tip end. The processing unit is configured forestimating a bending moment at a first estimation position along thelongitudinal axis based on the first sensor set signal, wherein thefirst sensor position is different from the first estimation positionalong the longitudinal axis.

There is a need for a wind turbine blade having an optical sensor systemwith low loss.

Accordingly, a wind turbine blade is provided, the wind turbine bladeextending along a longitudinal axis from a root end to a tip end and ina transverse plane perpendicular to the longitudinal axis, thetransverse plane having a main axis extending through an elastic centerpoint, the wind turbine blade comprising a blade shell forming aprofiled contour including a pressure side and a suction side, as wellas a leading edge and a trailing edge with a chord having a chord lengthextending there between, the main axis being parallel to the chord. Thewind turbine blade comprises a sensor system with an optical pathcomprising a first optical sensor fiber, a second optical sensor fiberand optionally a patch optical fiber, the first optical sensor fiberincluding a first core with a first core diameter, wherein the firstoptical sensor fiber extends from a first end to a second end andcomprising at least one sensor, the second optical sensor fiberincluding a second core with a second core diameter, wherein the secondoptical sensor fiber extends from a first end to a second end andcomprising at least one sensor. The patch optical fiber includes a patchcore with a patch core diameter, wherein the patch optical fiber extendsfrom a first end to a second end. The patch optical fiber may connectthe first optical sensor fiber and the second optical sensor fiber, andthe first core diameter may be the same as the patch core diameter.

The sensor system with a patch optical fiber provides a high degree ofdesign freedom in designing a wind turbine blade and provides an easilyadaptable optical sensor system that may be used in different easilyconfigurable configurations and wind turbine blade models.

Further, manufacture of the wind turbine blade may be facilitated sinceassembly of the sensor system does not require specialist knowledge ortools.

Further, the wind turbine blade according to the invention allows foreasy reconfiguration of the sensor system after moulding and assembly ofblade shell parts.

Wind turbine comprising a plurality of wind turbine blades including afirst wind turbine blade as described herein, wherein the wind turbinecomprises a blade monitoring system configured to estimate bendingmoment of the first wind turbine blade based on sensor set signals fromthe sensor system of the first wind turbine blade.

The present invention relates to a wind turbine blade, e.g. for a rotorof a wind turbine having a substantially horizontal rotor shaft, therotor comprising a hub, from which the blade extends substantially in aradial direction when mounted to the hub, the blade extending along alongitudinal axis from a root end to a tip end and in a transverse planeperpendicular to the longitudinal axis, the transverse plane having amain axis extending through an elastic center point. The wind turbineblade comprises a blade shell forming a profiled contour including apressure side and a suction side, as well as a leading edge and atrailing edge with a chord having a chord length extending therebetween. The profiled contour, when being impacted by incident airflow,generates a lift. The profiled contour is divided into: a root regionhaving a substantially circular or elliptical profile closest to theroot end with a root diameter being the chord length at the root end, anairfoil region having a lift-generating profile furthest away from theroot end, and a transition region between the root region and theairfoil region, the transition region having a profile graduallychanging in the radial direction from the circular or elliptical profileof the root region to the lift-generating profile of the airfoil region,and with a shoulder having a shoulder width and a shoulder distance andlocated at the boundary between the transition region and the airfoilregion, wherein the blade has a blade length. By shoulder is meant theposition at which the wind turbine blade has its largest chord lengthand the shoulder distance is the distance from the root end to theshoulder. The length interval is defined from the root end to the tipend, the root end thus being positioned at r=0 and the tip end beingpositioned at r=L along the longitudinal axis.

The blade may comprise a blade shell with a shell body. The shell bodymay for instance be assembled from a pressure side shell and a suctionside shell, which are adhered or bonded to each other near the leadingedge and near the trailing edge. In another embodiment, the shell ismanufactured via a one-shot process, e.g. via a closed, hollow mouldingmethod.

The shell body may comprise a longitudinally extending load carryingstructure, such as a main laminate. Such a load carrying structure ormain laminate is typically formed as a fibre insertion which comprises aplurality of fibre reinforcement layers, e.g. between 20 and 50 layers.On each side of the load carrying structure, the blade typicallycomprises a sandwich structure with a core material, such as balsa woodor foamed polymer, and with an inner and outer skin made of fibrereinforced polymer.

One or more sensors may be arranged in the main laminate or in edgesthereof, e.g. the first sensor axis of one or more sensor sets may crossthe main laminate or a main laminate edge.

The blade shell is typically made of a fibre reinforced polymermaterial. The reinforcement fibres may for instance be glass fibres,carbon fibres, aramid fibres, metallic fibres, such as steel fibres, orplant fibres, whereas the polymer for instance may be epoxy, polyesteror vinylester.

The wind turbine blade comprises a sensor set comprising at least onesensor, e.g for measuring or determining bending moments. A sensor setmay alternatively or in combination be configured for measuring otherparameters. The at least one sensor set includes a first sensor setpositioned at a first sensor position along the longitudinal axis. Thefirst sensor set may be positioned at a first distance d₁ from the rootend.

The wind turbine blade may comprise a plurality of sensors including thefirst sensor set and a second sensor set positioned at a second sensorposition along the longitudinal axis. The second sensor set may bepositioned at a second distance d₂ from the root end.

The plurality of sensor sets may include a third sensor set formeasuring a third bending moment in a third sensor position at a thirddistance d₃ from the root end.

The plurality of sensor sets may include a fourth sensor set formeasuring a fourth bending moment in a fourth sensor position at afourth distance d₄ from the root end.

The plurality of sensor sets may include a fifth sensor set formeasuring a fifth bending moment in a fifth sensor position at a fifthdistance d₅ from the root end.

The first distance d₁ may be in the range from about 1 m to about 20 mand the second distance d₂ may be in the range from about 3 m to about50 m.

Optionally, sensor position distances from the root end may depend onthe length L of the wind turbine blade and/or the shoulder distance orposition d₅.

The first distance d₁ may be selected in the range from d_(1,min) tod_(1,max). The first distance d₁ may be in the range from about 4 m toabout 15 m such as in the range from about 6 m to about 10 m, preferablyabout 8 m.

The first distance d₁ may depend on the shoulder distance d_(s), i.e. d₁may be a function of the shoulder distance d_(s). For example, d₁ may begiven byd ₁=α₁ d _(s),where a₁ is in the range from 0.2 to 1.0.

The first distance d₁ may depend on the length of the wind turbineblade, i.e. d₁ may be a function of the blade length L. For example, d₁may be given byd ₁=β₁ L,where β₁ is in the range from about 0.05 to about 0.95.]

The first distance d₁ may depend on the root diameter d_(root), i.e. d₁may be a function of the root diameter d_(root). The first distance d₁may be at least a diameter of the root.

For example, d₁ may be given byd ₁=γ₁ d _(root),where γ₁ is at least 0.8.

The minimum first distance d_(1,min) may depend on the root diameterd_(root), e.g. be given byd _(1,min)=γ₁ d _(root),where where γ₁ is at least 0.8, such as about 1.

The maximum first distance d_(1,max) may depend on the shoulder distanced_(s), e.g. be given byd _(1,max)=α₁ d _(s),where α₁ is in the range from 0.5 to 1.0.

The shoulder distances d_(s) may be in the range from 11 m to 15 m, e.g.about 13 m for a wind turbine blade having a length about 61.5 m.

The root diameter d_(root) may be in the range from 2 m to 5 m, e.g.about 2.5 m for a wind turbine blade having a length from 40 m-50 m, orabout 3.5 m for a wind turbine blade having a length from 60 m-75 m.

The second distance d₂ may be selected in the range from d_(2,min) tod_(2,max). The second distance d₂ may be in the range from about 5 m toabout 40 m, such as in the range from about 10 m to about 30 m such asabout 12 m or about 23 m.

The second distance d₂ may depend on the shoulder distance d_(s), i.e.d₂ may be a function of the shoulder distance d_(s). For example, d₂ maybe given byd ₂=α₂ d _(s),where α₂ is in the range from 0.5 to 10.0.

The second distance (d₂) may depend on the length of the wind turbineblade, i.e. d₂ may be a function of the blade length L. For example, d₂may be given byd ₂=β₂ L,where β₂ is in the range from 0.1 to 0.8.

The second distance d₂ may depend on the root diameter d_(root), i.e. d₂may be a function of the root diameter d_(root). For example, d₂ may begiven byd ₂=γ₂ d _(root),where γ₂ is at least 0.8.

The minimum second distance d_(2,min) may depend on the root diameterd_(root), e.g. be given byd _(2,min)=γ₂ d _(root),where γ₂ is at least 0.8, such as about 2.

The maximum second distance d_(2,max) may depend on the shoulderdistance d_(s), e.g. be given byd _(2,max)=α₂ d _(s),where α₂ is in the range from 0.5 to 3.0.

The distances d₁, d₂, d₃, . . . of sensor sets from the root end may beat least 6 m in order to minimize or avoid undesired non-linearitiesfrom pitch bearings in sensor measurements.

In one or more embodiments, the distances d₁, d₂, d₃, . . . of sensorsets from the root end may be less than 25 m, e.g. less than 20 m, toreduce or avoid excessive errors of superposition in sensormeasurements.

The distances between sensor sets along the longitudinal axis areselected in order to facilitate desired measurements, e.g. forestimating bending moments along the wind turbine blade such as in oneor more estimation positions. Distances between sensor positions areindicated as d_(ij), where i and j are index numbers for sensor sets andsensors thereof.

The distance d₁₂ between the first sensor set and optional second sensorset along the longitudinal axis may be in the range from about 1 m toabout 30 m, such as from about 3 m to about 20 m, e.g. about 4 m, about10 m, about 15 m.

The distance d₁₃ between the first sensor set and optional third sensorset along the longitudinal axis may be in the range from about 1 m toabout 50 m, such as from about 10 m to about 40 m, e.g. about 15 m,about 25 m, about 35 m.

The distance d₁₄ between the first sensor set and optional fourth sensorset along the longitudinal axis may be in the range from about 1 m toabout 60 m, such as from about 15 m to about 50 m, e.g. about 20 m,about 30 m, about 40 m.

The distance d₁₅ between the first sensor set and optional fifth sensorset along the longitudinal axis may be in the range from about 20 m toabout L-d₁, such as from about 20 m to about 60 m, e.g. about 30 m,about 40 m, about 50 m.

A sensor set may comprise one or more sensors. The one or more sensorsof a sensor set may include a primary sensor and optionally a secondarysensor. Sensor(s) of a sensor set may be adapted to measure bendingmoment components, i.e. a sensor set may comprise a primary sensor formeasuring a primary bending moment component M_(X) about a first axisperpendicular to the longitudinal axis, and/or a secondary sensor formeasuring a secondary bending moment component M_(Y) about a second axisperpendicular to the longitudinal axis. In one or more embodiments, asensor set consists of two sensors for measuring bending momentcomponents, thereby allowing bending moment measurements on the windturbine blade with only two sensors.

The wind turbine blade comprises a first sensor set, e.g. for measuringa first bending moment, in a first sensor position at a first distancefrom the root end.

The first sensor set may comprise a first primary sensor for measuring aprimary component of the first bending moment (M_(X,1)) about a firstaxis perpendicular to the longitudinal axis (first primary sensor) and afirst secondary sensor for measuring a secondary component of the firstbending moment (M_(Y,1)) about a second axis perpendicular to thelongitudinal axis (first secondary sensor).

The second sensor set may comprise a second primary sensor for measuringa primary component (M_(X,2)) of the second bending moment about a firstaxis perpendicular to the longitudinal axis (second primary sensor) anda second secondary sensor for measuring a secondary component of thesecond bending moment (M_(Y2)) about a second axis perpendicular to thelongitudinal axis (second secondary sensor).

Sensors of a sensor set are positioned at the same distance from theroot end, i.e. in the same transverse plane. In one or more embodiments,sensors of a sensor set, e.g. a primary sensor and a secondary sensor,may be displaced along the longitudinal axis. The distance betweensensors of a sensor set along the longitudinal axis should be as smallas possible. The maximum distance between sensors of a sensor set may beless than 1 m, such as less than 0.5 m. Larger distances may beemployed. In case of displaced sensors of a sensor set, the sensor setdistance to the root is the mean distance for sensors of the sensor set.

The length L of the wind turbine blade may be at least 40 m.

Root moments of a wind turbine blade are desired from a controlperspective of the wind turbine, e.g. in order to control pitch andother operating parameters in order to optimize operation and poweroutput of the wind turbine. However, measurement of the root moment withsensors positioned at the root end of the blade is largely affected bynon-linear moment contributions from the pitch bearing.

Correct positioning of sensors in a wind turbine blade is important inorder to obtain a precise measurement and in order to reduce oreliminate undesired effects such as nonlinear effects. It is from a costand manufacture perspective desired to employ a low number of sensors.

A primary sensor is positioned on a primary sensor axis extendingthrough the elastic center in the transverse plane with the primarysensor. A secondary sensor is positioned on a secondary sensor axisextending through the elastic center in the transverse plane with thesecondary sensor.

The angle between the first primary sensor axis and the first secondarysensor axis may be in the range from about 85° to about 95°. The anglebetween the first primary sensor axis and the first secondary sensoraxis may be about 90°.

In case of perpendicular first and second sensor axes, the first bendingmoment M₁ may be given as:M ₁=√{square root over (M _(X,1) ² +M _(Y,1) ²)}

An estimated primary component of the bending moment at the firstestimation position M_(X,est), e.g. at the root end, may be given as afunction of one or more sensor signals S₁₁, S₁₂, S₂₁, S₂₂, . . . , whereS₁₁ is the first primary sensor signal, S₁₂ is the first secondarysensor signal, S₂₁ is the second primary sensor signal, S₂₂ is thesecond secondary sensor signal, etc.

An estimated secondary component of the bending moment at the firstestimation position M_(Y,est), e.g. at the root end, may be given as afunction of one or more sensor signals S₁₁, S₁₂, S₂₁, S₂₂, . . . , whereS₁₁ is the first primary sensor signal, S₁₂ is the first secondarysensor signal, S₂₁ is the second primary sensor signal, S₂₂ is thesecond secondary sensor signal, etc.

Sensor signals may be multiplexed in time and/or infrequency/wavelength. Time multiplexing may be preferred for reducingthe number of components in the reading unit and reduce the costs of thesensing system.

The estimated primary component of the bending moment at the firstestimation position M_(x,est,1) may be estimated based only on primarysensor signals from primary sensors of one or more sensor sets. Theestimated secondary component of the bending moment at the firstestimation position M_(Y,est,1) may be estimated based only on secondarysensor signals from secondary sensors of one or more sensor sets.Additionally or alternatively, M_(x,est,1) may be estimated based onsecondary sensor signals from secondary sensors of one or more sensorsets and/or M_(Y,est,1) may be estimated based on primary sensor signalsfrom primary sensors of one or more sensor sets.

The bending moment M_(est,1) at a first estimation position may beestimated or given as:M _(est,)=√{square root over (M _(X,est,1) ² +M _(Y,est,1) ²)}.

The bending moment M_(est,1) at a first estimation position may beestimated or given as:M _(est,1)=√{square root over (α_(x) M _(X,est,1) ²+β_(Y) M _(Y,est,1)²)},where α_(X) and β_(Y) are compensation factors for compensating fornon-perpendicular first axis and second axis.

The wind turbine blade may comprise a second sensor set, e.g. formeasuring a second bending moment, in a second sensor position at asecond distance from the root end. The sensor set may comprise a secondprimary sensor for measuring a primary component and a second secondarysensor for measuring a secondary component, wherein a second primarysensor axis in the transverse plane is oriented in a direction definedby the second primary sensor and the elastic center point, and a secondsecondary sensor axis in the transverse plane is oriented in a directiondefined by the second secondary sensor and the elastic center point. Anangle between the second primary sensor axis and the second secondarysensor axis may be in the range from 50° to 130°, such as in the rangefrom 85° to 95°. The angle between the first primary sensor axis and thefirst secondary sensor axis may be about 90°.

The primary sensor(s), e.g. the first primary sensor and/or the secondprimary sensor, may for instance be arranged at the pressure side or thesuction side of the blade, advantageously at or embedded in a loadcarrying structure such as a main laminate of the blade shell structure.

The secondary sensor(s), e.g. the first secondary sensor and/or thesecond secondary sensor, may for instance be arranged at the leadingedge or the trailing edge of the wind turbine blade

Positioning primary and secondary sensors on the pressure side and atthe leading edge, respectively, may facilitate employment of straingauge sensors. One or more of the sensors, such as the primary and/orthe secondary sensors, may be strain gauge sensors.

One or more of the sensors, such as the primary and/or the secondarysensors, may be optical sensors, such as fiber Bragg gratings.

At least one of the sensors may be embedded in the blade shell. One ormore sensors may be attached to or mounted on the inner surface of theblade shell. The wind turbine blade may comprise a beam attached to theshell and at least one of the sensors may in this case be mounted on thebeam. One or more sensors may be mounted on or embedded in the mainlaminate of a shell body.

The wind turbine blade may comprise a web attached to the shell and atleast one of the sensors may be mounted on the web.

The one or more sensor sets, e.g. the first sensor set, and/or thesecond sensor set may be arranged in the root region, the transitionregion or the airfoil region of the wind turbine blade.

The sensors of the wind turbine blade provide sensor signals indicativeof a bending moment, e.g. in the form of strain/pressure signalsindicating strain/pressure applied to the sensor. The strain/pressuresignals of sensors can be transformed to bending moments or componentsthereof by use of sensor systems parameters, e.g. determined duringcalibration and/or design of the wind turbine blade.

A sensor signal indicative of a bending moment may be an optical signalfrom an optical sensor, e.g. a fiber Bragg grating, in an optical fiber,where the sensor signal is reflected light having a wavelength dependingon the strain applied to the sensor.

The sensor signal may be fed to a reading unit positioned e.g. in thewind turbine blade or in the hub of a wind turbine. The reading unit maybe configured to determine or derive the wavelength of one or moresensor signals and provide the wavelength(s) of sensor signal(s) to aprocessing unit in a blade monitoring system. The reading unit may be aseparate device connectable to a sensor system and a blade monitoringsystem for receiving sensor signals and forwarding processed sensorsignals to the blade monitoring system. In one or more embodiments, thereading unit may be embedded in the blade monitoring system, i.e. theblade monitoring system comprises the reading unit and is connectable tothe sensor system via one or more sensor ports of the interface.

In one or more embodiments where the sensors are implemented as fiberBragg gratings in an optical fiber, the sensor signals are opticalsignals with wavelengths λ₁₁ (first primary sensor), λ₁₂ (firstsecondary sensor), λ₂₁, (second primary sensor), λ₂₂ (second secondarysensor), . . . , λ_(ij) where i and j are index numbers for sensor setsand sensors thereof. The sensor signals may be multiplexed in timeand/or frequency/wavelength. The strain and/or pressure applied to thesensors may be derived from the wavelengths of the sensor signals. Theprocessing unit may be configured to derive bending moments and/orcomponents thereof from the sensor signals, e.g. based on sensor systemparameters stored in a memory unit.

For a wind turbine with an optical sensor system including a secondoptical sensor fiber and a patch optical fiber, the second core diametermay be the same. The optical path in the sensor system may have the samediameter, i.e. different optical fibers in the system may have the samecore diameter or mode field diameter.

The first optical sensor fiber may comprise a first primary sensor andoptionally a second primary sensor for indicating strain at a firstdistance and optionally at a second distance, respectively, from theroot end of the wind turbine blade.

The second optical sensor fiber may comprise a first secondary sensorand optionally a second secondary sensor for indicating strain at afirst distance and optionally at a second distance, respectively, fromthe root end of the wind turbine blade.

Standard patch optical fibers have a core diameter of 9 μm. A sensingfiber typically has a core diameter of less than 7 μm. Having the samecore diameter in the first optical sensor fiber and the patch opticalfiber provides an optical path with low damping and thus a sensor systemwith low loss is provided. An optical sensor system with low loss, e.g.less than 3.5 dB, may be provided in order to meet sensor systemrequirements from the reading unit or in order to reduce therequirements to the reading unit.

The sensor system may comprise a plurality of patch optical fibers withthe same patch core diameter.

The sensor system may comprise a number of optical fibers, each opticalfiber comprising one or more sensors, including a third optical fiberand/or a fourth optical fiber. The third optical fiber may be positionedparallel and adjacent to the first optical sensor fiber in the windturbine blade and/or the fourth optical fiber may be positioned paralleland adjacent to the second optical sensor fiber in the wind turbineblade. More than two optical fibers with sensors may allow for easyrepair of the sensor system. For example, if the first optical sensorfiber breaks, the patch optical fiber is simply connected to the thirdoptical fiber and the blade monitoring system is configured to estimatebending moments based on sensor signals from sensors in the second andthird optical fiber instead of estimating bending moments based onsensor signals from sensors in the first and second optical sensorfiber.

Provision of one or more patch cables with the same core diameters asthe optical fibers with sensor(s) facilitates a high degree of freedomfor the blade designer.

An optical fiber with a relatively small core diameter, e.g. less than 7μm, may be preferred for sensing in the wind turbine blade where a lowbend-induced loss may be of importance. A core diameter of 4.2 μm may beused since a high photosensitivity may be desirable in order tofacilitate fabrication of certain types of fiber Bragg gratings (FBGs).

Accordingly, the first core diameter, the second core diameter and/orthe patch core diameter may be less than 7 μm, such as 6.4 μm, 5.3 μm or4.2 μm.

The first and/or second optical sensor fibers may be single modefiber(s) with a design wavelength of 1550 nm.

The patch optical fiber(s) may be single mode fiber(s) with a designwavelength of 1550 nm.

In one or more embodiments, the first and second optical sensor fibersand the patch optical fiber are optical fibers having MFD of 6.4 μm andouter diameter of 80 μm.

In one or more embodiments the first and second optical sensor fibersand the patch optical fiber are optical fibers having MFD of 4.2 μm andouter diameter of 125 μm.

The optical fibers may have a cut-off wavelength in the range from 1350to 1500 nm.

The optical fibers may be SM 1500(4.2/125) optical fibers.

The distance between sensors of the sensor system may be at least 3 malong the optical path of the sensor system. In particular, the distancebetween the first primary sensor and the second primary sensor may be atleast 3 m, e.g. at least 5 m, at least 8 m, along the first opticalsensor fiber in order to enable sensing at distances along thelongitudinal axis of the wind turbine. The distance between the firstsecondary sensor and the second secondary sensor may be at least 3 malong the second optical sensor fiber, e.g. at least 5 m, at least 8 m.

An inter-sensor distance of at least 3 m facilitates time multiplexingof sensor signals. The distances between sensors in an optical fiber areselected in order to arrange sensors in desired positions in the windturbine blade.

The first optical sensor fiber and the second optical sensor fiber maybe mainly or at least partly embedded in the blade shell. Preferably,one or more ends of the first optical sensor fiber and/or the secondoptical sensor fiber are provided with connector parts for coupling orconnection to a patch optical fiber, a reading unit or another opticalfiber with sensors. The connector part(s) may be embedded oraccommodated in one or more connector boxes mounted on or moulded intothe blade shell for allowing easy installation of one or more patchoptical fibers. Connector parts at ends of the optical fibers provideoptical coupling of the fiber cores of the optical fibers.

The first optical sensor fiber and the second optical sensor fiber maybe at least partly adhered or mounted to the blade shell.

The wind turbine blade may comprise a reading unit comprising at leastone sensor port including a first sensor port for optically coupling thesensor system, e.g. the first optical sensor fiber, to the reading unit.The reading unit may be configured for reading sensor signals of opticalsensors in the sensor system as described above. The reading unit may beconfigured to derive a plurality of sensor signals indicative of strainon sensors of the sensor system in the wind turbine blade.

The reading unit may comprise a second sensor port, for example suchthat the second end of the second optical sensor fiber can be opticallycoupled to the second sensor port for reading sensor signals of opticalsensors in the sensor system. This configuration in combination withoptical coupling of the second end of the first optical sensor fiber andthe first end of the second optical sensor fiber enables monitoring ofsensors even in case of a damage at a single point in the optical pathformed by the first optical sensor fiber and the second optical sensorfiber.

The temperature may affect operation and characteristics of sensors,i.e. the sensor signals from the optical sensors may depend on thetemperature. A temperature-insensitive determination of bending momentmay be desired.

Accordingly, the sensor system may comprise a first temperature sensorin the first optical sensor fiber and/or a second temperature sensor inthe second optical sensor fiber. A fiber Bragg grating (FBG) of anoptical fiber may be arranged in the wind turbine blade such that nochanges in strain or pressure are applied to the FBG. Thereby the FBGmay function as a temperature sensor.

The first core diameter of the first optical sensor fiber may be equalto the second core diameter of the second optical sensor fiber.

The sensor system may comprise a beam splitting/combining unit having afirst, second and third port, wherein the first port is opticallycoupled to the first end of the first optical sensor fiber and thesecond port is optically coupled to the second end of the second opticalsensor fiber, such that sensor signals from the first optical sensorfiber are combined with sensor signals from the second optical sensorfiber on the third port connected to a reading unit.

The processing unit may be configured to compensate for temperaturevariation, i.e. estimation of bending moments may comprise applying acompensation factor to the sensor signals, the compensation factor beingbased on one or more temperature signals from the first temperaturesensor and/or the second temperature sensor.

Optical sensors are preferred in order to reduce or eliminate thedamages of lightning strike.

The first optical sensor fiber may comprise a first end connector partand/or a second end connector part for connecting the first end andsecond end, respectively, to a reading unit, a blade monitoring systemand/or other optical fibers.

The second optical sensor fiber may comprise a first end connector partand/or a second end connector part for connecting the first end andsecond end, respectively, to a reading unit, a blade monitoring systemand/or other optical fibers.

The connector parts may be an E2000 connector.

In the method for estimating bending moment of a wind turbine blade,estimating a bending moment at a first estimation position may becarried out assuming a zero bending moment at the tip end of the windturbine blade.

The distance between the first sensor position and the first estimationposition along the longitudinal axis may be at least 1 m, such as atleast 3 m, preferably in the range from 3 m to about 12 m.

During operation of a wind turbine, information on loads on the windturbine blade may be of interest from a turbine control viewpoint.Accordingly, information on root moments of wind turbine blade may be adesired parameter. The first estimation position may be any positionalong the longitudinal axis, such as at the root end of the wind turbineblade.

The first bending moment may have a primary component about a first axisperpendicular to the longitudinal axis and/or a secondary componentabout a second axis perpendicular to the longitudinal axis.

The method may comprise obtaining a second sensor set signal indicativeof a second bending moment at a second sensor position along thelongitudinal axis and estimating a bending moment at a first estimationposition may be based on the second sensor set signal.

In the method, the distance between the first sensor position and thesecond sensor position along the longitudinal axis may be at least 1 m.

The second bending moment may have a primary component about a firstaxis perpendicular to the longitudinal axis and a secondary componentabout a second axis perpendicular to the longitudinal axis.

The first axis and the second axis may be perpendicular or form anangle, e.g. the smallest angle may be in the range from 75° to about90°.

The first axis may be perpendicular to the primary sensor axis. Thesecond axis may be perpendicular to the secondary sensor axis.

In the method and in the blade monitoring system, estimating a bendingmoment may comprise estimating a primary component M_(X,est) about afirst axis perpendicular to the longitudinal axis and/or a secondarycomponent M_(Y,est) about a second axis perpendicular to thelongitudinal axis at a first estimation position and/or at a secondestimation position along the longitudinal axis. Estimating a bendingmoment may comprise curve fitting.

In the method and in the blade monitoring system, the bending momentM_(est,1) or components thereof M_(X,est,1) and/or M_(Y,est,1) at thefirst estimation position may be estimated by using a firstapproximation function from the tip end to the first sensor position anda second approximation function from the first sensor position to thefirst estimation position. The second approximation function may bebased on the first approximation function. Different approximationfunctions may be employed for primary and secondary components, i.e. afirst and second primary approximation function may be employed for theprimary component and a first and second secondary approximationfunction may be employed for the secondary component

The first approximation function may be selected from a cubic splinefunction and a polynomial function. The polynomial function may be afirst order, a second order, a third order, a fourth or higher orderpolynomial function. The second approximation function may be a linearinterpolation.

The first sensor position may be located in the transition region or theairfoil region of the wind turbine blade.

The method may comprise transmitting the estimated bending moment orcomponents thereof to a control system of a wind turbine, e.g. a blademonitoring system, pitch control system, a wind turbine controller, awind park controller, alarm system or the like.

The blade monitoring system may be configured for implementing one ormore parts or steps of the method described herein. The blade monitoringsystem enables determination and estimation of bending moments orcomponents thereof applied to one or more wind turbine blades of a windturbine, e.g. during operation of the wind turbine. Thus, the inventionallows for a control system of the wind turbine to adjust operationalparameters such as pitch angles in order to optimize power productionand lifetime of the wind turbine and wind turbine blades.

The blade monitoring system may comprise a memory unit connected to theprocessing unit, the memory unit being configured for storing sensorsystem parameters of the sensor system. The processing unit may beconfigured for estimating a bending moment based on sensor systemparameters stored in the memory unit, e.g. sensor system parametersderived during calibration or manufacture of the wind turbine blade.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become readily apparent to those skilled in the art by thefollowing detailed description of exemplary embodiments thereof withreference to the attached drawings, in which:

FIG. 1 illustrates a wind turbine,

FIG. 2 illustrates a wind turbine blade,

FIG. 3 is a cross section of a wind turbine blade,

FIG. 4 illustrates different views of a wind turbine blade,

FIG. 5 is a flow diagram of an exemplary method according to theinvention,

FIG. 6 illustrates a cross section of a wind turbine blade,

FIG. 7 illustrates a cross section of a wind turbine blade,

FIG. 8 illustrates a cross section of a wind turbine blade,

FIG. 9 illustrates a cross section of a wind turbine blade,

FIG. 10 illustrates a cross section of a wind turbine blade,

FIG. 11 illustrates a wind turbine blade with sensor system according tothe invention,

FIG. 12 schematically illustrates a first optical sensor fiber and apatch optical fiber,

FIG. 13 illustrates a blade monitoring system,

FIG. 14 illustrates a blade monitoring system, and

FIG. 15 illustrates estimated bending moment with curve fitting.

DETAILED DESCRIPTION OF THE INVENTION

The figures are schematic and simplified for clarity, and they merelyshow details which are essential to the understanding of the invention,while other details have been left out. Throughout, the same referencenumerals are used for identical or corresponding parts.

FIG. 1 illustrates a conventional modern upwind wind turbine accordingto the so-called “Danish concept” with a tower 4, a nacelle 6 and arotor with a substantially horizontal rotor shaft. The rotor includes ahub 8 and three blades 10 extending radially from the hub 8, each havinga blade root 16 nearest the hub and a blade tip 14 furthest from the hub8. The rotor has a radius denoted R.

FIG. 2 shows a schematic view of a first embodiment of a wind turbineblade 10 according to the invention. The wind turbine blade 10 has theshape of a conventional wind turbine blade and comprises a root region30 closest to the hub, a profiled or an airfoil region 34 furthest awayfrom the hub and a transition region 32 between the root region 30 andthe airfoil region 34. The blade 10 comprises a leading edge 18 facingthe direction of rotation of the blade 10, when the blade is mounted onthe hub, and a trailing edge 20 facing the opposite direction of theleading edge 18.

The airfoil region 34 (also called the profiled region) has an ideal oralmost ideal blade shape with respect to generating lift, whereas theroot region 30 due to structural considerations has a substantiallycircular or elliptical cross-section, which for instance makes it easierand safer to mount the blade 10 to the hub. The diameter (or the chord)of the root region 30 may be constant along the entire root area 30. Thetransition region 32 has a transitional profile gradually changing fromthe circular or elliptical shape of the root region 30 to the airfoilprofile of the airfoil region 34. The chord length of the transitionregion 32 typically increases with increasing distance r from the hub.The airfoil region 34 has an airfoil profile with a chord extendingbetween the leading edge 18 and the trailing edge 20 of the blade 10.The width of the chord in the airfoil region decreases with increasingdistance r from the hub.

A shoulder 40 of the blade 10 is defined as the position, where theblade 10 has its largest chord length. The shoulder 40 is typicallyprovided at the boundary between the transition region 32 and theairfoil region 34.

The blade 10 has different airfoil profiles 41, 42, 43, 44, 45, 46 alongthe longitudinal axis of the blade.

As illustrated in FIG. 4, the wind turbine blade 10 comprises at leastone sensor set including a first sensor set positioned at a firstposition along the longitudinal axis. The first sensor set comprises afirst primary sensor 47A and optionally a first secondary sensor 47Bpositioned at a first distance d₁ from the root end. The sensor 47A andthe sensor 47B may be displaced a distance d_(1,12) along thelongitudinal direction. The distance d_(1,12) may be less than 1 m.

Optionally, the wind turbine blade 10 comprises a second sensor setpositioned at a second position along the longitudinal axis. The secondsensor set comprises a second primary sensor 48A and optionally a secondsecondary sensor 48B positioned at a second distance d₂ from the rootend. The sensor 48A and the sensor 48B may be displaced a distanced_(2,12) along the longitudinal direction. The distance d_(2,12) may beless than 1 m.

In the wind turbine blade 10, the sensors are optical sensors in theform of an optical fiber with fiber Bragg gratings embedded in the shellof the wind turbine blade. The sensors of the wind turbine blade may bea part of the same optical fiber and/or be a part of different opticalfiber sections coupled by one or more optical connectors.

It should be noted that the chords of different sections of the bladenormally do not lie in a common plane, since the blade may be twistedand/or curved (i.e. pre-bent), thus providing the chord plane with acorrespondingly twisted and/or curved course, this being most often thecase in order to compensate for the local velocity of the blade beingdependent on the radius from the hub.

Table 1 below illustrates different suitable combinations of sensorpositions (distances from the root end), optionally dependent on thelength of the wind turbine blade.

TABLE 1 Sensor positions. d₁/m d₂*/m d₃*/m d₄*/m d₅*/m L/m 2-L  3-L10-L  20-L 30-L ≧40 2-20  3-40 10-45 4-15  5-30 4-10 10-30 6-10 10-1515-20 5 10 8 12 23 40 50 ≧60 1.5*d_(root) 0.9*d_(s) 8 d₁ + d₁₂ 8 12 8 2340 *if present

Sensor position configuration may depend on the number of sensor setsavailable and estimation position(s). A sensor position near the rootend may be desirable, however a sensor position too near the root end isnot desirable due to contributions or noise from the pitch bearings.

FIGS. 3 and 4 depict parameters, which may be used to explain thegeometry of the wind turbine blade according to the invention.

FIG. 3 shows a schematic view of an airfoil profile 50 of a typicalblade of a wind turbine depicted with the various parameters, which aretypically used to define the geometrical shape of an airfoil. Theairfoil profile 50 has a pressure side 52 and a suction side 54, whichduring use—i.e. during rotation of the rotor—normally face towards thewindward (or upwind) side and the leeward (or downwind) side,respectively. The airfoil 50 has a chord 60 with a chord length cextending between a leading edge 56 and a trailing edge 58 of the blade.The airfoil 50 has a thickness t, which is defined as the distancebetween the pressure side 52 and the suction side 54. The thickness t ofthe airfoil varies along the chord 60. The deviation from a symmetricalprofile is given by a camber line 62, which is a median line through theairfoil profile 50. The median line can be found by drawing inscribedcircles from the leading edge 56 to the trailing edge 58. The medianline follows the centres of these inscribed circles and the deviation ordistance from the chord 60 is called the camber f. The asymmetry canalso be defined by use of parameters called the upper camber (or suctionside camber) and lower camber (or pressure side camber), which aredefined as the distances from the chord 60 and the suction side 54 andpressure side 52, respectively.

Airfoil profiles are often characterised by the following parameters:the chord length c, the maximum camber f, the position d_(f) of themaximum camber f, the maximum airfoil thickness t, which is the largestdiameter of the inscribed circles along the median camber line 62, theposition d_(t) of the maximum thickness t, and a nose radius (notshown). These parameters are typically defined as ratios to the chordlength c. Thus, a local relative blade thickness t/c is given as theratio between the local maximum thickness t and the local chord lengthc. Further, the position d_(p) of the maximum pressure side camber maybe used as a design parameter, and of course also the position of themaximum suction side camber.

FIG. 4 shows other geometric parameters of the blade. The blade has atotal blade length L. As shown in FIG. 3, the root end is located atposition r=0, and the tip end located at r=L. The shoulder 40 of theblade is located at a position r=d_(s), and has a shoulder width W,which equals the chord length at the shoulder 40. The diameter of theroot is defined as d_(root). The curvature of the trailing edge of theblade in the transition region may be defined by two parameters, viz. aminimum outer curvature radius r_(o) and a minimum inner curvatureradius r_(i), which are defined as the minimum curvature radius of thetrailing edge, seen from the outside (or behind the trailing edge), andthe minimum curvature radius, seen from the inside (or in front of thetrailing edge), respectively. Further, the blade is provided with apre-bend, which is defined as Δy, which corresponds to the out of planedeflection from a pitch axis 22 of the blade.

FIG. 5 illustrates an exemplary method according to the presentinvention. The method 100 comprises obtaining 102 a first sensor setsignal indicative of a first bending moment at a first sensor positionalong the longitudinal axis of the wind turbine blade. Further, themethod 100 comprises estimating 104 a bending moment at a firstestimation position along the longitudinal axis based on the firstsensor set signal, wherein the first sensor position is different fromthe first estimation position along the longitudinal axis. The methodmay be employed on a wind turbine blade as described herein. Optionally,the method 100 comprises obtaining 106 a second sensor set signalindicative of a second bending moment at a second sensor position alongthe longitudinal axis, and estimating 104 a bending moment is based onthe second sensor set signal. Sensor signals may be obtained seriallyand/or in parallel.

The first sensor set signal comprises a first primary sensor signal froma first primary sensor (47A) and a first secondary sensor signal from afirst secondary sensor (47B). The first primary sensor signal indicatesa primary component M_(x,1) of the first bending moment and the firstsecondary sensor signal indicates a secondary component M_(y,1) of thefirst bending moment.

FIG. 6 and FIG. 7 are cross sections illustrating examples of sensor setpositioning on a wind turbine blade.

In FIG. 6, the wind turbine blade 10′ comprises a first sensor set witha first primary sensor 47A and a second primary sensor 47B at a firstdistance d₁ from the root. The first primary sensor 47A lies on a firstprimary sensor axis 74 extending through the elastic center 70 of theblade cross section (transverse plane at first distance d₁). The firstsecondary sensor 47B lies on a first secondary sensor axis 76 extendingthrough the elastic center 70 of the blade cross section. In FIG. 6, theelastic center lies on the chord 60 and thus the main axis 72 coincidewith the chord 60. The angle α₁ between the two sensor axes 74 and 76 is90°. The angle β₁ between the main axis 72 and the first primary sensoraxis 74 is 90°.

In FIG. 7, the wind turbine blade 10″ comprises a first sensor set witha first primary sensor 47A and a second primary sensor 47B at a firstdistance d₁ from the root. The first primary sensor 47A lies on a firstprimary sensor axis 74 extending through the elastic center 70 of theblade cross section (transverse plane at first distance d₁). The firstsecondary sensor 47B lies on a first secondary sensor axis 76 extendingthrough the elastic center 70 of the blade cross section. In FIG. 7, theelastic center lies on the chord 60 and thus the main axis 72 coincidewith the chord 60. The angle α₁ between the two sensor axes 74 and 76 is90°. The angle β₁ between the main axis 72 and the first primary sensoraxis 74 is 75°.

In FIG. 6 and FIG. 7, the sensors are embedded in the shell body 78 ofthe wind turbine blade.

FIG. 8 is a cross section illustrating an example of sensor setpositioning on a wind turbine blade. The wind turbine blade 10′″comprises a beam 80 attached to the shell body 78 and the second primarysensor 47B is attached to the beam 80 nearest the leading edge 56.

FIG. 9 is a cross section illustrating an example of first sensor setpositioning on a wind turbine blade at a first distance d₁. The windturbine blade comprises a shell body 78 and the sensors 47A, 47B aremounted on the inner surface of the shell body 78. The angle α₁ betweensensor axes 74 and 76 is 90°.

FIG. 10 is a cross section illustrating an example of second sensor setpositioning on a wind turbine blade at a second distance d₂. The windturbine blade comprises a shell body 78 and the sensors 48A, 48B aremounted on the inner surface of the shell body 78. The angle α₂ betweensensor axes 74 and 76 is 90°.

FIG. 11 illustrates a part of a wind turbine. The wind turbine comprisesa hub 8 from which the blades whereof a first wind turbine blade 10 isshown extend substantially in a radial direction when mounted to the hub8. The wind turbine blade 10 comprises a sensor system 82 with anoptical path comprising a first optical sensor fiber 84, a secondoptical sensor fiber 86 and a patch optical fiber 88. Optical connector90 couples the first optical sensor fiber 84 and the patch optical fiber88 and optical connector 90′ couples the second optical sensor fiber 86to the patch optical fiber 88. The optical fibers 84, 86, 88 are SM1500(4.2/125) fibers. The first optical sensor fiber comprises first primarysensor 47A and second primary sensor 48A in the form of fiber Bragggratings and optionally first temperature sensor 98A. The second opticalsensor fiber comprises first secondary sensor 47B and second secondarysensor 48B in the form of fiber Bragg gratings and optionally secondtemperature sensor 98B. The first end 85 of the first optical sensorfiber 84 is coupled to a reading unit 92 for reading sensor signals fromthe sensor system 82. The reading unit 92 provides wavelength values ofthe sensor signals to a blade monitoring system 94 via a data cable 96.The blade monitoring system is configured for estimating components ofthe bending moment at the root end of the wind turbine blade based onthe sensor signals and configured for transmitting the estimated bendingmoment to a turbine controller (not shown). The second end 85′ of thefirst optical connector 84 is optically coupled to the first end 89 ofthe patch optical fiber 88 in connector or connector assembly 90. Thesecond end 89′ of the patch optical fiber 88 is optically coupled to thefirst end 87 of the second optical sensor fiber 86 in connector orconnector assembly 90′.

FIG. 12 schematically illustrates the optical connectors or connectorassemblies 90 between the first optical sensor fiber 84 and the patchfiber 88. The first optical sensor fiber 84 comprises a first core 130with a first core diameter d_(core,1). The patch optical fiber 88comprises a patch core 132 with a patch core diameterd_(core,p)=d_(core,1)=4.2 μm. Fiber cladding material and sheet 134, 136protect the cores 130, 131, 132. The first optical sensor fiber 84comprises a first end connector part at the first end (not shown) and asecond end connector part 138 (e.g. female E2000 connector) at thesecond end 85′, and the patch optical fiber comprises a first endconnector part 140 (e.g. male E2000 connector) for connecting the firstoptical sensor fiber 84 and the patch optical fiber 88. The connectorassembly 90′ is formed in the same way as the connector assembly 90indicated by the reference numbers.

The second optical sensor fiber 86 includes a second core 131 with asecond core diameter d_(core,2)=d_(core,p), wherein the second opticalsensor fiber extends from a first end to a second end and comprising atleast one sensor. The second optical sensor fiber 86 comprises a firstend connector part 138′ (e.g. female E2000 connector) at the first end87 and a second end connector part (not shown) at the second end, andthe patch optical fiber comprises a second end connector part 140′ (e.g.male E2000 connector) for connecting the second optical sensor fiber 86and the patch optical fiber 88.

FIG. 13 schematically illustrates a blade monitoring system 94. Theblade monitoring system 94 comprises a housing 95 accommodating aprocessing unit 150 connected to an interface 152 and a memory unit 154via connections 155, 155′, respectively. The interface 152 comprises afirst connector port 156 and a second connector port 158. The firstconnector port 156 is configured for connection to a reading unit forreceiving data of sensor signals from a sensor system of a wind turbineblade. The second connector port 158 is configured for connection to aturbine controller for transmitting and/or sending data and/orcontrol/alarm signals to a turbine controller.

The processing unit 150 is configured for receiving a first sensor setsignal indicative of a first bending moment at a first sensor positionof a wind turbine blade extending along a longitudinal axis from a rootend to a tip end via the first connector port 156. Further, theprocessing unit 150 is configured for estimating a bending moment orcomponents thereof at a first estimation position along the longitudinalaxis based on the first sensor set signal, wherein the first sensorposition is different from the first estimation position along thelongitudinal axis.

FIG. 14 schematically illustrates a blade monitoring system 94′ whereina reading unit 92 is integrated in the blade monitoring system andconnected to the processing unit 150 via connection 155′″. The interface152 comprises a first connector port 156 in the form of a first sensorport 160 for coupling the sensor system, e.g. the first optical sensorfiber 84, (optionally via a patch optical fiber) to the reading unit 92of the blade monitoring system 94′.

FIG. 15 schematically illustrates estimation of a primary componentM_(X,est,1) of bending moment at the root end (first estimationposition) of a wind turbine blade having L=53.2 m. The first distance d₁is 7 m and the second distance d₂ is 10.5 m. The primary componentM_(X,est,1) is estimated based on a first primary sensor signal S₁₁ froma first primary sensor at d₁ and a second primary sensor signal S₂₁ froma second primary sensor at d₂.

It has been shown that using cubic spline functions from the firstprimary sensor to the tip end of the wind turbine blade and then usingthe gradient or derivative of the bending moment at the first distanceto perform a linear extrapolation from d₁ to the root end may bepreferred. Gradients of the bending moment at the tip end are zero.

Further, the bending moment gradient at d₁ may be estimated based on thebending moments at d₁ and d₂, e.g. with a backward Euler method. A largedistance between the first and second sensor sets may not be desirable.

The estimation as illustrated in FIG. 15 comprises the following stepsusing measured bending moment components at d₁ and d₂:

-   -   A first cubic spline function with “correct” boundary conditions        using the two measurement points and the tip end point where the        bending moment is zero is fitted used to perform the        interpolation of a point near the end of the wind turbine blade.        Here “correct” means that the first derivatives at the two ends        of the interval are correct,    -   the bending moment gradient at d₁ is estimated using the        measured sensor data transformed to bending moments at d₁        (M_(X,1)) and d₂ (M_(X,2)) with a backward Euler method, and the        derivative at the tip end is zero. The three points, i.e. d₁, d₂        and the tip end point are denoted points A in FIG. 15. The        interpolated point near the end of the wind turbine blade is        denoted point B.    -   Then a second spline function, covering the interval from d₁ to        the tip end is constructed using a “not-a-knot” method where the        points A and B are used.    -   Then, the second spline function is extended to the root end        with a linear extrapolation denoted app. A′ between d₁ and the        root end.

The “not-a-knot” method means that the third order derivative at thesecond and second last point of the domain used for interpolation is thesame when looking from each side of the point.

FIG. 15 illustrates estimations of the bending moment. In the graph,app. A′ represents sensor positions at d₁=7 m and d₂=10.5 m, and app. B′represents sensor positions at d₁=7 m and d₂=20 m. As can be seen onFIG. 15, app A′ provides a better estimation of primary component nearthe root end while app B′ provides a better estimation between 10 m and50 m from the root end.

It should be noted that in addition to the exemplary embodiments of theinvention shown in the accompanying drawings, the invention may beembodied in different forms and should not be construed as limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete, and will fullyconvey the concept of the invention to those skilled in the art.

LIST OF REFERENCE NUMERALS

-   2 wind turbine-   4 tower-   6 nacelle-   8 hub-   10, 10′, 10″,-   10′″ wind turbine blade-   15 blade tip-   16 blade root-   18 leading edge-   20 trailing edge-   22 pitch axis-   30 root region-   32 transition region-   34 airfoil region-   40 shoulder-   41, 42, 43,-   44, 45, 46 airfoil profile-   47A first primary sensor-   47B first secondary sensor-   48A second primary sensor-   48B second secondary sensor-   50 airfoil profile-   52 pressure side-   54 suction side-   56 leading edge-   58 trailing edge-   60 chord-   62 camber line/median line-   70 elastic center-   72 main axis-   74 primary sensor axis-   76 secondary sensor axis-   78 shell body-   80 beam-   82 sensor system-   84 first optical sensor fiber-   85 first end of first optical sensor fiber-   85′ second end of first optical sensor fiber-   86 second optical sensor fiber-   87 first end of second optical sensor fiber-   87′ second end of second optical sensor fiber-   88 patch optical fiber-   89 first end of patch optical fiber-   89′ second end of patch optical fiber-   90, 90′ optical connector-   92 reading unit-   94 blade monitoring system-   94′ blade monitoring system-   95 housing-   96 data cable-   98A first temperature sensor-   98B second temperature sensor-   99 beam splitting/combining unit-   130 first core-   131 second core-   132 patch core-   134 fiber cladding material and sheet-   136 fiber cladding material and sheet-   138 second end connector part of first optical sensor fiber-   138′ first end connector part of second optical sensor fiber-   140 first end connector part of patch optical fiber-   140′ second end connector part of patch optical fiber-   150 processing unit-   152 interface-   154 memory unit-   155, 155′,155″,-   155′″, 155″″ connection-   156 first connector port-   158 second connector port-   160 sensor port-   c chord length-   d_(t) position of maximum thickness-   d_(f) position of maximum camber-   d_(p) position of maximum pressure side camber-   d_(s) shoulder distance-   d_(root) root diameter-   f camber-   L blade length-   P power output-   r local radius, radial distance from blade root-   t thickness-   v_(w) wind speed-   θ twist, pitch-   Δy prebend-   α₁ angle between first primary sensor axis and first secondary    sensor axis-   α₂ angle between second primary sensor axis and second secondary    sensor axis.-   β₁ angle between first primary sensor axis and main axis-   β₂ angle between second primary sensor axis and main axis-   d_(core,1) first core diameter-   d_(core,2) second core diameter-   d_(core,p) patch core diameter

The invention claimed is:
 1. Wind turbine blade extending along alongitudinal axis from a root end to a tip end and in a transverse planeperpendicular to the longitudinal axis, the transverse plane having amain axis extending through an elastic center point, the wind turbineblade having a blade shell forming a profiled contour including apressure side and a suction side, and a leading edge and a trailing edgewith a chord having a chord length extending there between, the mainaxis being parallel to the chord, the wind turbine blade comprising: asensor system with an optical path having: a first optical sensor fiber,the first optical sensor fiber including a first core with a first corediameter, wherein the first optical sensor fiber extends from a firstend to a second end and having at least one sensor; a second opticalsensor fiber, the second optical sensor fiber including a second corewith a second core diameter, wherein the second optical sensor fiberextends from a first end to a second end and having at least one sensor;and a patch optical fiber, the patch optical fiber including a patchcore with a patch core diameter, wherein the patch optical fiber extendsfrom a first end to a second end and connects the first optical sensorfiber and the second optical sensor fiber, and wherein the first corediameter is the same as the patch core diameter.
 2. Wind turbine bladeaccording to claim 1, wherein the second core diameter is the same asthe patch core diameter.
 3. Wind turbine blade according to claim 1,wherein the first optical sensor fiber comprises a first primary sensorand a second primary sensor for indicating strain at a first distanceand at a second distance, respectively, from the root end of the windturbine blade.
 4. Wind turbine blade according to claim 1, wherein thesensor system comprises a first temperature sensor in the first opticalsensor fiber and a second temperature sensor in the second opticalsensor fiber.
 5. Wind turbine blade according to claim 1, wherein thesecond optical sensor fiber comprises a first secondary sensor and asecond secondary sensor for indicating strain at a first distance and ata second distance, respectively, from the root end of the wind turbineblade.
 6. Wind turbine blade according to claim 1, wherein the firstcore diameter is less than 7 μm.
 7. Wind turbine blade according toclaim 1, wherein the first and second optical sensor fibers is a singlemode fiber with a design wavelength of 1550 nm.
 8. Wind turbine bladeaccording to claim 1, wherein the distance between sensors of the sensorsystem is at least 3 m along the optical path of the sensor system. 9.Wind turbine blade according to claim 1, wherein the first opticalsensor fiber and the second optical sensor fiber are at least partlyembedded in the blade shell.
 10. Wind turbine blade according to claim1, wherein the first optical sensor fiber and the second optical sensorfiber are at least partly adhered or mounted to the blade shell. 11.Wind turbine blade according to claim 1, comprising a reading unitcomprising at least one sensor port including a first sensor port foroptically coupling the sensor system to the reading unit configured forreading sensor signals of optical sensors in the sensor system.
 12. Windturbine blade according to claim 11, wherein the reading unit isconfigured to derive a plurality of sensor signals indicative of strainon sensors in the wind turbine blade.
 13. Wind turbine comprising aplurality of wind turbine blades including a first wind turbine bladeaccording to claim 1, wherein the wind turbine comprises a blademonitoring system configured to estimate bending moment of the firstwind turbine blade based on sensor signals from the sensor system of thefirst wind turbine blade.